The sound level measured in the good state is predominantly caused by plant noises which for example are coupled in via pipe connections. These can change after calibration. If they become larger there is a possibility of a false alarm being triggered. If the plant noises become smaller during the course of operation this leads to a reduction in measurement sensitivity since the threshold value predetermined was too high. In addition the measurement of the sound level in the good state involves additional expenditure during commissioning. With operating conditions which vary greatly for the valve under investigation the diagnostic method cannot be employed because the plant sounds are heavily dependent on the operating conditions. For check valves of positive displacement pumps especially the method cannot be used under these circumstances since measurements have shown that the system noise always depends on the operating pressure and this rises by around 20 dB if there is a change in pressure from 5 to 50 bar.
An underlying object of the invention is to create a diagnostic system and a diagnostic method which react less sensitively to changes in the noise of the system.
To achieve this object the new diagnostic system of the type mentioned at the start has the features mentioned in the claims or the diagnostic method has the features mentioned in the claims. Advantageous developments are described in the dependent claims.
For a reliable detection of a leakage the leakage noise for a closed valve and the leakage noise for an open file are recorded almost simultaneously and compared with one another. The two measurement times can be all the closer to one another the shorter the gap is between the two states “valve open” and “valve closed”. In the special case of an application of the diagnostic system to check valves in oscillating positive displacement pumps the valves are periodically opened and closed. Since the two check valves assume it precisely opposite states at specific times the leakage noise can be recorded at one valve while at precisely the same time the plant noise is recorded at the other valve. The noises can be assigned to the relevant valve state in a simple way by suitable signal processing. In principle the diagnosis can however be performed in this case with just one solid-borne sound sensor which then records the leakage noise and the plant noise in the closed or open state of the same valve respectively in chronological order. However in this case the new diagnostic system has the advantage that it is largely insensitive to gradual changes in operating conditions.
Correspondingly the new diagnostic system and a new diagnostic method can be applied for valves in piston compressors which operate in accordance with a largely similar principle.
The diagnostic system has an especially low sensitivity to wide variations in operating conditions if the first value and the second value of the characteristic variable which are compared with each other are determined on the basis of the sound signal last recorded in the closed or the open state. In this case it can be assumed that the plant's noises in the intervening period have in any event only changed slightly and thereby barely affect the diagnosis result in any way.
In an application of the system to diagnosis of a check valve of a positive displacement pump it is advantageously sufficient to have especially low expenditure for measuring system components if the evaluation device is embodied such the respective state of the valve can be determined by it on the basis of the recorded sound signal. In this case no further means for recording the position of the closing element is necessary. The system can make do with just one sound recorder for a measuring point.